The present invention relates to an optical duobinary transmitter system and method using optical intensity modulation.
At high bit rates, the chromatic dispersion in standard single mode fibers (SSMF) limits the transmission distance in the 1550 nm window. There has been a number of different methods proposed to overcome this limitation of which the most common are pre-chirped modulators, dispersion compensating fibers, chirped Bragg gratings, mid-span spectral inversion, and special signal formats such as dispersion supported transmission and duobinary transmission.
Duobinary transmission has been investigated for modulators showing no or very little chirp, i.e. xcex1≈0, see, e.g. Gu et al., 10 Gbit/s unrepeatered three-level optical transmission over 100 km of standard fiber, Electron. Lett., Vol. 29, No. 25, 1993, pp. 2209-2211 and May et al., Extended 10 Gbit/s fiber transmission distance at 1538 nm using a duobinary receiver, IEEE Phot. Technol. Lett., Vol. 6, No. 5, 1994, pp 648-650. The chirp parameter xcex1 is defined as   α  =                    ∂        ϕ                    ∂        t                            1                  2          ⁢          P                    ⁢                        ∂          P                          ∂          t                    
where xcfx86 is the phase and P the intensity of the optical signal.
The duobinary signal is DC-free and its transmission spectrum is narrower than the spectrum of the binary signal. If the duobinary signal is modulated on a carrier, the modulated signal will behave as a double sideband signal with suppressed carrier.
The main benefit with duobinary transmission is that the transmission spectrum is reduced compared to ordinary binary transmission. In a dispersion limited system, the transfer length is inversely proportional to the square of the bandwidth of the transmission spectrum. This means that if the transmission spectrum is reduced to one half the transfer length is quadrupled.
Further, since the carrier frequency is suppressed in the duobinary transmission spectrum, the limitation for the output optical power due to stimulated Brillouin scattering in the fiber can be relaxed.
Optical duobinary transmission can be considered as a three-level signaling scheme which can be detected with an ordinary binary receiver. The normal marks in binary transmission are xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d, whether the marks in duobinary transmission are xe2x80x9cxe2x88x921xe2x80x9d, xe2x80x9c0xe2x80x9d, and xe2x80x9c1xe2x80x9d, In the optical case, the duobinary marks are modulated as xe2x80x9cxe2x88x92Pxe2x80x9d, xe2x80x9c0xe2x80x9d, and xe2x80x9cPxe2x80x9d, where P is the optical peak power. These will be interpreted as xe2x80x9cPxe2x80x9d, xe2x80x9c0xe2x80x9d, and xe2x80x9cPxe2x80x9d in an ordinary opto-electric quadratic detector.
A common way to construct an optical duobinary transmitter is to make use of a double-electrode Mach-Zehnder (DEMZ) modulator, see, e.g. the U.S. Pat. No. 5,543,952 or the international application WO 95/29539. The DEMZ-modulator has also been proposed for adjustable chirp applications, see A. H. Gnauck et al., Dispersion penalty reduction using an optical modulator with adjustable chirp, IEEE Phot. Technol. Lett., Vol. 3, No. 10, 1991, pp 916-918, as well as simultaneous 2:1 multiplexing and modulation, see P. B. Hansen et al., A dual-drive Ti:LiNbO3 Mach-Zehnder Modulator used as an optoelectric logic gate for 10 Gbit/s simultaneous multiplexing and modulation, IEEE Phot. Technol. Lett., Vol. 4, No. 6, 1992, pp 592-593.
A typical optical duobinary transmitter based on a DEMZ-modulator according to prior art is explained with reference to the layout as shown in FIG. 1.
The input signal of the transmitter is an electrical binary signal S1 and its complement S2={overscore (S1)}. Each of these signals is fed through a binary-to-duobinary encoder 1, 3 and an AC-amplifier 5, 7. The resulting duobinary, i.e. three-level, signals S3, S4 are amplified and then used as driving signals of the electrodes of the modulator 9.
Continuous light from a laser diode 11 is coupled into the modulator 9 and split into two components in the Y-junction 9a of the left part of the modulator. The light in the two branches 9b, 9c of the modulator will then undergo positive or negative phase shift in the middle part of the modulator, the phase shift being controlled through the linear electro-optic effect by the applied voltage, i.e. the duobinary driving signals S3, S4, of the electrodes of the modulator. The phase shift in the upper branch is controlled by the upper electrode, and the phase shift in the lower branch is controlled by the lower electrode. The electrodes are supplied by bias voltage 13 in order to obtain the same phase shift in the two branches when no driving signals are applied to the electrodes.
The light in the two branches are then combined coherently in the Y-junction 9d in the right part of the modulator. If there is a 0xc2x0 phase shift between the components, all light will be injected in the outgoing optical waveguide. If there is a 180xc2x0 phase shift, no light will be injected in the outgoing waveguide. In the latter case, the light will be radiated into the modulator.
The coding procedure for duobinary transmission is very simple. In FIG. 2 is shown the binary-to-duobinary encoder 1 which converts the binary signal S1 into a duobinary signal S3 by using two flip-flops 15, 17 and a clock pulse 19. The flip-flops have binary output signals S5, 56, which are equal to the input binary signal but shifted one bit and two bits, respectively. The binary output signals S5, S6 are then fed through an adder 21 with the following function
S3=S5+S6xe2x88x921
thus, generating the duobinary signal S3. In FIG. 3 is shown an example of the output signal- S3 and the encoding intermediate signals S5, S6 for duobinary modulation of the binary signal S1. It may be observed that a direct transition between the marks xe2x80x9cxe2x88x921xe2x80x9d and xe2x80x9c1xe2x80x9d never occurs in duobinary modulation. The binary-to-duobinary encoder 3 is constructed and functioning likewise with the only difference that the input signal S2 is the complement of the binary signal S1.
The introduced phase shift in the upper and in the lower branch of the optical duobinary modulator for each of the marks are indicated in FIG. 4a. The logical xe2x80x9c1xe2x80x9d mark corresponds to a light pulse with full amplitude and a 0xc2x0 phase shift, the xe2x80x9c0xe2x80x9d mark corresponds to no light pulse at all as the two components are opposite in phase and cancel each other out, and the xe2x80x9cxe2x88x921xe2x80x9d mark corresponds to a light pulse with full amplitude and a 180xc2x0 phase shift.
FIG. 4b shows a polar graph (amplitude vs phase) of the locus of the optical output signal (thick solid line) and the location of each of the duobinary marks (dots). The phase of the optical output signal does not vary on its way between the marks. Therefore, dxcfx86/dt=0 and xcex1=0 according to the formula presented above.
The main problem with a duobinary transmitter as described is that the chromatic dispersion still limits the transmission distance and may be a problem for long haul fiber transmission systems.
An object of the present invention is to provide an optical duobinary transmitter with an improved performance in terms of dispersion immunity.
This object among others is fulfilled by an inventive optical duobinary transmitter system and method, which introduces a blue-shift frequency chirp.
The inventive system and method comprises an input terminal, a driving circuit, a double electrode optical modulator, particularly of the Mach-Zehnder type, and an output terminal.
The input terminal is arranged to receive a first binary signal and the driving circuit, which is connected to said input terminal, is arranged to convert the first binary signal into a second and a third binary signal. The double electrode optical modulator is connected to the driving circuit in such a way that its upper and lower electrode may be driven by said second and third binary signal, respectively, said modulator being further arranged to modulate the amplitude and phase of an optical carrier according to the binary driving signals so as to provide an optical duobinary signal corresponding to said first binary signal and with a predetermined negative modulation chirp parameter. Finally, the output terminal, which is connected to the optical modulator, is arranged to feed an optical transmission line with the modulated optical duobinary signal.
Preferably, the driving circuit comprises a first and a second logical gate whose outputs are connected to the respective electrode of the double electrode optical modulator. The logical gates may be an AND- or a NAND-gate and an OR- or a NOR-gate, respectively.
The logical gates are driven by two binary signals that may be the outputs of either a demultiplexer or two flip-flops, which in turn is/are driven by the first binary signal.
The demultiplexer would be arranged to demultiplex the first binary signal, e.g. ABCDEFGH, into two binary signals, e.g. AACCEEGG* and *BBDDFFHH, respectively, where * denotes an undefined signal mark.
The two flip-flops would be serially connected and arranged to demultiplex the first binary signal, e.g. ABCDEFGH, into two binary signals, e.g. *ABCDEFGH and **ABCDEFGH, respectively.
Furthermore, the second and the third binary signals may be arranged to be amplified prior to driving the electrodes of the modulator.
The double electrode optical modulator is preferably arranged to introduce the same phase shift of the optical carrier components led through the two branches for a given applied voltage. The three optical duobinary marks may be provided as no light pulse (or a light pulse with a very low amplitude), a first light pulse with a high amplitude and a second light pulse with a high amplitude, the two latter light pulses being opposite in phase to each other.
The double electrode optical modulator may further be arranged to provide modulated light with a phase y having a negative time derivative, i.e. dxcfx86/dt less than 0, when the intensity of the modulated light is being raised.
In another embodiment of the present invention the upper and lower electrodes of the modulator are driven by a first and a second quasi-ternary signal. Preferably, the first and second quasi-ternary signals are provided with non-equidistant marks, e.g. xe2x80x9c1xe2x80x9d, xe2x80x9c0.25xe2x80x9d, xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d, xe2x80x9c0.75xe2x80x9d, xe2x80x9c0xe2x80x9d, respectively, in order to obtain a predetermined amount of negative chirp, e.g. xcex13dB=xe2x88x920.5.
An advantage of the invention is that it may into some extent compensate for dispersion in dispersive systems such as a fiber-optic system operating at 1550 nm in standard single mode fibers.
Another advantage is that it needs a lower receiver sensitivity for a given transmission distance as compared with the transmitter system described in prior art.
Still another advantage of the invention is that when the AC-amplifiers amplify binary signals instead of duobinary signals the demands on them are relaxed.
Yet another advantage is that the invention is easy and simple to implement and uses a minimum of coding electronics.